adolescent

Where it comes from

Adolescent ordinarily means a young person developing from childhood into adulthood. Adolescent came through French or directly from Latin adolescens, a young person and the present participle of adolescere, to grow up or come to maturity. The verb is related to alere, to nourish, and also supplied the Latin participle adultus, grown up. The Latin grammar captures a contrast English has obscured: an adolescent is growing toward maturity, while an adult is the grown-up result.

How the meaning developed

English borrowed adolescent as a noun centuries before its common adjective use.

In use

One is still growing up. The other is the grown-up result.
Latin grammar made that contrast clearer than English does.